I have to use STO-nG (a "minimal basis set" - meaning that only one basis function is used for each atomic orbital in the atoms of which the molecule is made from). Let's take the example of a water molecule.

The water molecule has two H atoms and one O atom. Thus, we have a total of 7 orbitals (two 1s of H, one 1s of O, one 2s of O and three 2p of O). So when using STO-nG, would that then mean that three 1s type basis functions, one 2s type basis function, and 3 p-type basis functions are used, totalling at 7 basis functions, each being made up from a linear combination (LC) of n simple gaussians?

  • 3
    $\begingroup$ Possible duplicate of How many basis functions used in STO-3G and 6-31+G** for the water molecule? $\endgroup$
    – Tyberius
    Commented Mar 27, 2019 at 14:11
  • $\begingroup$ @Tyberius Yes. But if you see the answers, this has not been answered. $\endgroup$ Commented Mar 27, 2019 at 16:34
  • 1
    $\begingroup$ Sorry I missed that they never explicitly answered that. But you are correct, STO-nG for water will have 7 basis functions, each of which is formed from n-Gaussians. As an aside, you could always check this using a free electronic structure program like Psi4 or ORCA. @hhsomething69 $\endgroup$
    – Tyberius
    Commented Mar 27, 2019 at 16:50

1 Answer 1


Yes, you have counted the number of basis functions correctly:

    #1s    #2s    #2px    #2py    #2pz    #Total
H1   1                                      1
H2   1                                      1
O    1      1       1       1       1       5

Which as you also counted is 7 basis functions. As you also correctly state, each of the basis functions will be a linear combination of $n$ primitive Gaussians:


As you have observed that is what the $n$ in STO-$n$G denotes.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.